Method of injecting molten metal into a mold cavity

ABSTRACT

A method of injecting molten plastic or aluminum material into a cavity defined within a mold to form a desired part shape is provided. The method includes: (a) providing pressurized gas in the mold cavity prior to injection of molten material into the cavity; (b) generating a melt pressure sufficient to inject the molten material into the cavity; and (c) providing a closed loop controller to maintain a controllable difference between the melt pressure and the pressure of gas in the cavity throughout a substantial portion of a period of time during which molten material is being injected into the cavity, the difference being sufficient to inject molten material into the cavity in a manner to improve the conditions under which the material solidifies. By preloading the system with static pressure, conditions are created which allow generation, measurement and real time control of pressure difference profiles during transition of the melt throughout the injection cycle.

RELATED APPLICATION

This application is a continuation-in-part of U.S. patent applicationSer. No. 08/236,471, filed May 2, 1994, now U.S. Pat. No. 5,441,680,herein incorporated by reference in its entirety.

TECHNICAL FIELD

The present invention relates to a method of injecting molten materialinto a cavity defined within a mold to form a desired part shape.

BACKGROUND ART

When molten plastic is processed by an injection molding machine, theplastic enters a mold cavity where it is cooled to form a desired partshape. As the cooling occurs, the plastic contracts within the cavity.As a result of this contraction, the part actually shrinks in size, andsink marks or low spots often occur on the surface of the part. Shrinkand sink marks have caused major problems for injection molders sinceinjection molding was first developed. Several methods have beendeveloped in an attempt to eliminate these problems. Some examplesinclude gas-assisted injection molding, structural foam molding, liquidgas assisted molding, etc. In addition, foaming agents have been used inthe molding process for mixing with molten plastic in order to generateinert gases in the plastic. These gases provide internal pressure in theplastic which enables the plastic to more fully fill the cavity of themold and packs the plastic against the cavity walls. This, in turn,helps reduce sink on the surface of the plastic parts. Also, gascounterpressure in the mold cavity has been used to improve surfacesmoothness of molded parts.

These prior art methods are all problematic due to the large number ofvariables in the molding process. Varying injection pressures andinjection speeds, varying melt pressures and temperatures, varyingcavity conditions, and uncontrolled venting of gases all contribute toan unstable molding environment. These various problems in the moldingprocess create burning and scission of polymer chains and createinternal stresses within the plastic which remain in the plastic as theplastic material cools in the cavity. These internal stresses causeshrink, sink, and warpage of the plastic part to be molded. In addition,these various molding problems lead to degradation of the plasticmaterial as it is processed through an injection molding machine. Ingeneral, erratic variations in pressure, temperature, and injectionspeed create material breakdown and cause internal problems in theplastic which show up in the final product as molded.

Another disadvantage of prior art systems is that the plastic melt flowin these systems faces changes in pressure due to changes in cavitygeometry as the molten plastic moves into the cavity of the mold. Thesepressure changes cause certain areas of the cavity to be filled morequickly then other areas, thus resulting in different coolingcharacteristics in different areas of the cavity. These coolingvariations cause inconsistency in the direction of plasticsolidification, which results in surface stresses, weld lines or sink.

It is desirable to develop a more balanced injection molding process inwhich the pressure of the molten plastic is more tightly and evenlycontrolled as the plastic moves through the injection molding machine.It is further desirable to develop an injection molding process in whichpressures acting upon the plastic are balanced in order to eliminate theabove referenced problems caused by variations in polymer chainconditions, in order to reduce internal stresses in the plastic. Theultimate goal of such an injection molding process would be to produce afinal product which nearly perfectly matches the cavity surface of themold and is fully relieved of internal stresses which lead to shrink,sink and warpage thereof and has greatly improved mechanical properties.In addition, part weight may be reduced, which will provide significantmaterial savings to the manufacturers.

In aluminum casting, the primary problem experienced by manufacturers isthe formation of hydrogen voids and gas porosity in the aluminum as thealuminum is cast. Many of these problems arise due to the fact that themold cavity cannot be perfectly and completely evacuated of gases. Thegas porosity and voids in the aluminum adversely effect metallic bondingand, accordingly, adversely effect mechanical properties of the finalproduct. Furthermore, voids and gas porosity may create stresses withinthe part which can create warpage, decrease mechanical strength of thepart, and cause dimensional variations in the part. Another problemfaced by manufacturers is that formation of waves in the sleeve as theplunger moves forward may lead to entrapment of gases within the moltenaluminum, which causes the same problems as described above. Waves mayalso be generated within the mold cavity as the material is injectedinto the mold. It would be desirable to develop a method of aluminumcasting in which the solubility of hydrogen in the molten aluminum isincreased and controlled in order to avoid the above referencedproblems. It would further be desirable to develop a method of aluminumcasting in which the formation of waves in the sleeve is decreased andthe venting power of the liquid metal is increased.

DISCLOSURE OF THE INVENTION

This invention stems from the realization that, when injecting moltenmetal or molten plastic into a mold cavity, it is desirable to preloadthe system with pressure, which provides conditions under which pressurechanges become measurable, and controllable pressure differences may beestablished between the pressure of gas in the mold cavity and thepressure of the molten material. By providing real-time closed loopcontrol, the gas pressure and static melt pressure of the moltenmaterial may be sensed and mathematically monitored by the controller inorder to provide optimal pressure conditions for injection andsolidification of the material in the mold cavity. This closed looppressure control on the basis of pressure differences created frompre-programmed transition of a preloaded melt into a mold cavityprovides the capability to control the static pressure of the meltthroughout the injection and solidification cycle and to provide optimalinjection and solidification pressure conditions for the melt as themelt moves from the melt holder and solidifies within the mold cavity.

A method of injection molding is provided for use with an injectionmolding machine, comprising: (a) generating internal counterpressurewithin molten plastic as plastic pellets are plasticized in theinjection molding machine; and (b) pressurizing air within a cavity of amold in the injection molding machine to an air pressure level which issubstantially equal to the internal counterpressure in order tocounterbalance the internal counterpressure as the molten plastic isinjected into the cavity, thus providing a substantiallypressure-balanced molding environment for the plastic.

Further provided for use with an injection molding machine is a methodof injection molding comprising: (a) generating pressurized gas andpressurized moisture within molten plastic as plastic pellets areplasticized in the injection molding machine, the pressurized gas andthe pressurized moisture having a total pressure defining an internalcounterpressure within the molten plastic; (b) pressurizing air within acavity of a mold in the injection molding machine to an air pressurelevel which is substantially equal to the internal counterpressure inorder to counterbalance the internal counterpressure as the moltenplastic is injected into the cavity, thus providing a substantiallypressure-balanced molding environment for the plastic; and (c)maintaining the air pressure level in the cavity substantially constantas the molten plastic is injected into the cavity.

Also provided for use with an injection molding machine is a method ofinjection molding, comprising: (a) calculating a maximum stress to beexperienced by a shot of plastic to be molded in the injection moldingmachine, the stress being a result of a volumetric shrink occurring asthe plastic is cooled in a cavity of a mold in the machine; (b)pressurizing a shot of plastic to a first melt pressure as the plasticis plasticized in a barrel of the injection molding machine, the firstmelt pressure being substantially equal to the calculated maximumstress; (c) pressurizing air within the cavity to an air pressuresubstantially equal to the first melt pressure; (d) commencing injectionof the shot of plastic into the cavity in a laminar flow manner, whereinmolten plastic flows into said vicinity concentrically with respect to apoint at which plastic enters the cavity; (e) increasing the meltpressure on the shot of plastic to a second melt pressure, whilemaintaining the air pressure within the cavity substantially constant,and maintaining a substantially constant difference between the airpressure within the cavity and the second melt pressure during asubstantial portion of a period of time which the shot of plastic isbeing injected into the cavity; (f) sensing the first and second meltpressures and generating feedback signals indicative thereof; (g)receiving said feedback signals, comparing said feedback signals toreference values, and producing signals for controlling said first andsecond melt pressures; and (h) repeating steps (b-g).

Also provided is a method of injection molding a plastic part for usewith an injection molding machine, comprising: (a) forming a pluralityof vents in a mold for use in the injection molding machine, the moldhaving a cavity formed therein, the vents being in fluid flowcommunication with the cavity of the mold to vent pressurized air fromthe cavity, while maintaining a substantially constant air pressure inthe cavity, according to the following formula: ##EQU1## where A is across-sectional area of the vent, W is discharge of air through the ventin pounds per second, C is a coefficient of flow, P1 is the air pressurein the cavity in pounds per square inch, and T1 is a temperature in thecavity in degrees Fahrenheit;

(b) forming a channel in the mold in fluid flow communication with thevents; (c) sealing the mold to prevent leakage of pressurized air fromthe cavity; (d) providing first and second valves in selective fluidflow communication with the channel formed by the mold, the first valvebeing selectively movable between a closed position wherein pressurizedair is prevented from moving therethrough and an open position whereinpressurized air is allowed to enter the channel therethrough, and thesecond valve being selectively movable between a closed position whereinpressurized air is prevented from moving therethrough and an openposition wherein pressurized air is allowed to discharge therethroughfrom the channel; (e) calculating a maximum stress to be experienced bya shot of plastic to be molded in the injection molding machine, thestress being a result of volumetric shrink occurring as the plastic iscooled in the cavity of the mold; (f) pressurizing a shot of moltenplastic in a barrel of the injection molding machine to a first meltpressure, the first melt pressure being substantially equal to themaximum stress; (g) moving the first valve to the open position; (h)moving the second valve to the closed position; (i) introducingpressurized air through the first valve into the cavity until the airpressure in the cavity is equal to the first melt pressure; (j) movingthe first valve to the closed position; (k) injecting the shot ofplastic into the cavity; (l) increasing the melt pressure of the shot ofplastic to a second melt pressure, thus creating a pressure differencebetween the second melt pressure and the gas pressure in the cavity; (m)maintaining the pressure difference between the second melt pressure andthe air pressure in the cavity substantially constant for a substantialportion of a period of time in which plastic is being injected into thecavity; (n) moving the second vent to the open position to releasepressurized air therethrough; and (o) repeating steps (f)-(n).

Further provided is a mold for use in an injection molding machine,comprising a front half and a back half of the mold. The front halfincludes an aperture formed therethrough for receiving molten plasticfrom the injection molding machine. The front half and back halfcooperate to form a cavity therebetween, and the cavity is in fluid flowcommunication with the aperture to receive molten plastic therefrom. Aplurality of vents are formed in one of the back half and front half,the vents having first and second ends thereof. The first end of each ofthe plurality of vents is in fluid flow communication with the cavity.The vents are configured according to the following formula to maintaina substantially constant air pressure in the cavity as the cavity isbeing filled with plastic: A=0.24241 , W*√T1/(C*P1), where A is across-sectional area of a vent, W is discharge of pressurized airthrough the vent in pounds per second, C is a coefficient of flow, P1 isthe air pressure in the cavity in pounds per square inch, and T1 is atemperature in the cavity in degrees Fahrenheit. A channel is formed inone of the back half and the front half, the channel being in fluid flowcommunication with the second end of each of the plurality of vents fortransferring pressurized air into and out of the cavity through thevents. A pair of valves are provided in selective fluid flowcommunication with the channel. One of the pair of valves is adapted toselectively receive pressurized air from a pneumatic line to providepressurized air to the channel, and the other of the pair of valves isadapted to selectively allow discharge of pressurized air from thechannel. A seal is circumscribed around the cavity and positionedbetween the front half and back half to prevent discharge of pressurizedair from the cavity between the front half and the back half of the moldas the cavity is being filled with molten plastic.

Also provided is a method of reducing internal stresses in plastic partsformed in a mold cavity from molten plastic injected into the moldcavity by an injection molding apparatus, comprising the steps of:pressuring the cavity to a predetermined air pressure; operating theinjection molding apparatus to develop molten plastic at a first meltpressure equal to the predetermined air pressure; communicating themolten plastic with the mold cavity when the predetermined air pressureand first melt pressure become equal; and subsequently increasing themelt pressure to a second melt pressure, and maintaining a substantiallyconstant difference between the air pressure within the mold cavity andthe second melt pressure during a substantial portion of a predeterminedperiod of time in which the molten plastic is being injected into themold cavity, whereby to optimize pressure conditions acting upon themolten plastic in a manner to reduce internal stresses in the plasticparts being formed.

Further provided is a method of injection molding for use with aninjection molding machine including a mold with a cavity formed thereinfor receiving molten plastic and a hydraulic unit for creating aninjection pressure to fill the mold cavity with molten plastic at apredetermined melt pressure, comprising: supplying air to the cavity ata predetermined air pressure; sensing the melt pressure duringinjection; sensing the air pressure in the cavity during injection; andproviding a closed loop controller to monitor the sensed melt pressureand sensed air pressure and to produce signals to be sent to thehydraulic unit for maintaining the melt pressure at desired levels.

Also provided is a method of injection molding for use with an injectionmolding machine including a mold therein, the method comprising:determining a maximum stress to be experienced by plastic as the plasticis processed in the mold; generating counterpressure within the plasticprior to injection of the plastic into the mold, said counterpressurebeing substantially equal to said determined maximum stress; andmaintaining said counterpressure within the plastic at least equal tothe determined maximum stress as the plastic is injected into the mold.

Further provided is a method of injection molding for use with aninjection molding machine including a mold with a cavity formed thereinfor receiving pressurized air and means for creating an injectionpressure sufficient to fill the mold cavity with molten plastic, themolten plastic having a total pressure defining a melt pressure, themethod comprising: monitoring the pressure of air in the mold cavity;monitoring the injection pressure while the mold cavity is being filled;and maintaining a substantially constant difference between the pressureof air in the cavity and the injection pressure while molten plastic isbeing injected into the cavity to optimize molding conditions.

In addition, a method of injecting molten metal into a mold cavitydefined within a mold in a metal casting apparatus is provided,comprising: supplying pressurized gas to the mold cavity and melt holderto pressurize the gas in the cavity and molten metal in the melt holderto provide a preloaded static system which creates conditions underwhich any pressure differences from following transitions are measurableand are used for closed loop control during injection; generating a meltpressure in the molten metal sufficient to inject the molten metal intothe cavity after the cavity is pressurized; measuring the melt pressureand the pressure of gas in the cavity as the melt moves from the meltholder into the mold cavity; and providing a closed loop controller tomaintain a predetermined controllable pressure difference between themelt pressure and the pressure of gas in the cavity throughout asubstantial portion of a period of time during which molten metal isbeing injected into the cavity to fill the cavity.

Also provided is a method of metal casting without the formation ofsignificant porosity and voids for use with a metal casting apparatusincluding a sealable sleeve and a mold having a mold cavity formedtherein, the sleeve having a plunger for pushing molten metal from thesleeve into the mold cavity in a potentially wave-forming manner, theplunger being movable from a first position in which the plunger isretracted, a second position at which injection of the molten metal intothe mold cavity begins and a third position at which the mold cavity issubstantially full of molten metal, the method comprising: supplyingpressurized gas in the sleeve adjacent the molten metal to sufficientlyincrease the level of solubility of hydrogen in the molten metal as theplunger is moved from the first position toward the second position,whereby to prevent formation of significant gas porosity and voids inthe casting and decrease wave formation; and supplying pressuring gas inthe mold cavity to sufficiently increase the level of solubility ofhydrogen in the molten metal in the mold cavity as the plunger is movedfrom the second position toward the third position, whereby to preventformation of significant porosity and voids in the casting.

Further provided is a method for metal casting without the formation ofsignificant gas porosity and voids for use with a metal castingapparatus including a sealable sleeve and a mold having a mold cavityformed therein, the sleeve having a plunger for pushing molten metalfrom the sleeve into the mold cavity, the plunger being movable from afirst position at which the plunger is retracted, to a second positionat which the injection of the molten metal into the mold cavity begins,and to a third position at which the mold cavity is substantially fullof molten metal, the method comprising: providing pressurized gas in themold cavity and in the sleeve to increase the level of solubility ofhydrogen in the molten metal as the molten metal travels from the sleeveinto the mold cavity, whereby to prevent the formation of significantgas porosity and voids.

Another aspect of the present invention provides a method of aluminumcasting for use with an aluminum casting apparatus including a sleeveand a mold having a mold cavity formed therein, the sleeve having aplunger for pushing molten aluminum from the sleeve into the moldcavity, the molten aluminum having a static pressure and a dynamicpressure during movement into the cavity, the method comprising:providing pressurized gas in the mold cavity sufficient to generate andmaintain a static pressure in the molten aluminum as the molten aluminummoves from the sleeve into the mold cavity, the static pressure beingsufficient to maintain an increased level of solubility of hydrogen inthe molten aluminum as the molten aluminum travels from the sleeve intothe mold cavity.

Further provided is a method of aluminum casting for use with analuminum casting apparatus including a sleeve and a mold having a moldcavity formed therein, the sleeve having a plunger for pushing moltenaluminum from the sleeve into the mold cavity in a manner potentiallyforming volumetric deficits and negative pressures in the moltenaluminum as the aluminum solidifies in the cavity, the molten aluminumhaving a static pressure and a dynamic pressure during movement into thecavity, the method comprising: providing pressurized gas in the moldcavity sufficient to generate and maintain a static pressure in themolten aluminum as the molten aluminum moves from the sleeve into themold cavity, said static pressure being sufficient to maintain anincreased level of solubility of hydrogen in the molten aluminum and toreduce volumetric deficits and negative pressures formed in the moltenaluminum as the molten aluminum travels from the sleeve and solidifiesin the mold cavity.

Also provided is a method of metal casting for use with a metal castingapparatus including a sleeve and a mold having a mold cavity formedtherein, the sleeve having a sealable feed throat and a plunger forpushing molten metal from the sleeve into the mold cavity in apotentially wave forming manner, the plunger being movable from a firstposition at which the plunger is retracted, to a second position atwhich injection of the molten metal into the mold cavity begins, and toa third position at which the mold cavity is substantially full ofmolten metal, the method comprising: providing pressurized gas in thesleeve adjacent the molten metal sufficient to decrease the amplitude ofany waves formed in the molten metal as the plunger moves from the firstposition toward the second position, whereby to discourage entrapment ofgases in the molten metal.

Further provided is a method of improving venting power of molten metalas it is injected from a melt holder into a mold cavity formed within amold in a metal casting apparatus. The cavity includes smallindentations formed therein, such as ribs or bosses, which exist lateralto the flow direction of the molten metal. The method comprises:providing pressurized gas to the mold cavity; and maintaining a pressuredifference between the dynamic melt pressure and the pressure of gas inthe cavity to create a static pressure in the molten metal, whereby toimprove the venting power of the molten metal to allow the molten metalto vent, fill and pack out any said indentations as the molten metalmoves through the cavity.

Also provided is a method of injecting molten metal from a melt holderinto a mold cavity defined within a mold in a metal casting apparatus,comprising: supplying pressurized gas to the mold cavity and to the meltholder to pressurize gas in the cavity and molten metal in the meltholder sufficiently to provide a preloaded static system which createsconditions under which any pressure differences from followingtransitions are measurable and are used for closed loop control duringinjection; generating a melt pressure in the molten metal sufficient toinject the molten metal into the cavity after the gas in the cavity ispressurized; measuring the melt pressure and the pressure of gas in thecavity as the melt moves from the melt holder into the mold cavity; andproviding closed loop pressure control based upon pressure differencescreated from preprogrammed transition of the preloaded molten metal intothe cavity, whereby to create and control a static pressure of themolten metal throughout injection and solidification of the metal withinthe cavity.

Further provided is a method of improving feeding of solidifying crystalstructures of molten metal as the molten metal is cast in a metalcasting apparatus, the apparatus including a sleeve and a mold having amold cavity formed therein, the sleeve having a plunger for pushingmolten metal from the sleeve into the mold cavity, the plunger beingmovable between a first position at which the plunger is retracted, asecond position at which injection of the molten metal in the moldcavity begins, and a third position at which the mold cavity issubstantially full of molten metal, the method comprising: supplyingpressurized gas in the sleeve adjacent the molten metal as the plungeris moved from the first position toward the second position; andsupplying pressurized gas in the mold cavity sufficiently to improvefeeding of the molten aluminum into the solidifying crystal structuresin order to reduce formation of voids and to improve metallic bondingduring solidification.

Also provided is a method of eliminating the effects of mold resistanceresulting from varying gate configurations and mold cavityconfigurations in a metal casting process for use with a metal castingapparatus including a sealable sleeve and a mold having a mold cavityformed therein, the sleeve having a plunger for injecting molten metalfrom the sleeve into the mold cavity, the method comprising providingpressurized gas in the sleeve prior to injection of the molten metalfrom the sleeve into the cavity and providing pressurized gas in thecavity as the molten metal is injected into the cavity, said pressurizedgas in the cavity having a gas pressure at least as great as anypressure drops caused by said varying gate configurations and moldcavity configurations, whereby to eliminate mold resistance effects ofvarying cavity configurations and gate configurations.

These and other features, objects and advantages of the presentinvention will become apparent upon reading the following descriptiontherefor together with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic of an injection unit for an injection moldingmachine having a mold and pressure control system connected theretoaccording to the present invention;

FIG. 2 is plan view of a back half of a mold according to the presentinvention;

FIG. 3 is a vertical cross-sectional view taken through FIG. 2 of aninjection mold according to the present invention;

FIG. 4 is a graphical illustration of a melt pressure and air pressurecycle in an injection molding machine according to the presentinvention;

FIG. 5 is a graphical illustration of a pressure difference profilebetween a melt pressure and an air pressure in an injection moldingmachine according to the present invention;

FIG. 6 is a schematic view of an aluminum casting unit for an aluminumcasting apparatus having a mold and a pressure control system connectedthereto in accordance with the present invention;

FIG. 7 is a vertical cross-section of an aluminum casting apparatus,including a mold, with the molten aluminum in a position just prior toentry in the mold cavity in accordance with the present invention;

FIG. 8 is a vertical cross-section of an aluminum casting apparatusincluding a mold, with the molten aluminum completely filling the moldcavity in accordance with the present invention;

FIG. 9 is a graphical illustration of the melt pressure, shot speed andcavity air pressure versus time in accordance with the presentinvention;

FIG. 10 is a graphical illustration of the pressure difference betweenthe air pressure in the cavity and the melt pressure versus time;

FIG. 11 is a graphical illustration of the prior art melt pressure incomparison to the melt pressure and cavity pressure versus time inaccordance with the present invention; and

FIG. 12 is a graphical illustration of an injection power curve inaccordance with the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

Referring to FIG. 1, an injection molding machine 10 is shown, includingan injection unit 12, for use with the present method. Plastic resinmoves from the hopper 14 into the barrel 16 of the injection moldingmachine. Heat from the barrel 16 and rotational movement of the screw 18cause the plastic resin to melt and form a shot of plastic to be moldedby the machine. The shot of plastic is pressurized by the machine. Themelt pressure of the shot of plastic is measured and regulated through amelt pressure transducer 20. A positive shutoff valve is provided at thetip of the barrel 16 in order to prevent drool of plastic through thenozzle and to allow pressurization of molten plastic in the barrel 16.

A mold 22 is inserted into the injection molding machine 10. The mold 22includes a front half 24 and back half 26. The front half 24 has anaperture 28 formed therethrough for receiving the shot of molten plasticfrom the injection molding machine. The front half 24 and back half 26of the mold 22 cooperate to form a cavity 30 therebetween. The cavity 30is in fluid flow communication with the aperture 28 for receiving theshot of molten plastic therethrough. The shot of molten plastic ispacked into the cavity 30 and held therein in order to cool and form aplastic part matching the shape of the cavity.

Transducers are provided for sensing pressures throughout the moldingprocess. Injection pressure for the injection unit 12 is monitored bythe injection pressure transducer 32. Air pressure is provided to thecavity 30 of the mold 22 through the pneumatic line 34. Air pressure inthe cavity 30 is monitored by the air pressure transducer 36. The airpressure transducer is located in a vent so that a true pressure readingmay be taken from the cavity. The vent with the pressure transducer willbe discommunicated from the channel 44. A rubber seal 38 is providedbetween the front half and the back half 26 of the mold 22 to preventescape of pressurized air from the cavity 30 of the mold. Often, whenmolten plastic is injected into a mold under high pressure, the fronthalf 24 and back half 26 of a mold will separate slightly, thus allowingescape of pressurized air therefrom. The rubber seal 38 is designed toprevent this escape of pressurized air from the cavity.

A closed loop controller 40 is provided with the injection moldingmachine 10. The closed loop controller 40 receives pressure signals fromthe pressure transducers 20,32,36, compares these pressure signals topreviously determined reference values, and sends signals to theinjection molding machine to adjust the pressures accordingly.

Referring to FIG. 2, a plurality of vents 42 are shown in fluid flowcommunication with the cavity 30. The purpose of these vents is to allowdischarge of pressurized air and gases as molten plastic is injectedinto the cavity 30. A channel 44 is provided around the cavity 30 influid flow communication with the vents 42. The pressurized air andgases move through the vents into the channel. A first valve 46 isprovided in selective fluid flow communication with the channel 44 forproviding pressurized air from the pneumatic line 34 to the channel 44.Similarly, a second valve 48 is provided in selective fluid flowcommunication with the channel 44 for release of pressurized airtherefrom. The purpose of this sealed and valved venting system is toprovide venting orifice controls immediately in front of the melt flow,rather than outside of the mold. In this manner, cavity air pressure maybe provided to resist the melt pressure as molten plastic is injectedinto the cavity.

A method according to the present invention for use with the abovedescribed apparatus is based upon the fact that each volumetric unit ofmolten plastic injected into the cavity will shrink due to adjustment ofsurface tension forces during cooling, and substantial stresses will bebuilt up in the solid part. These conditions may be alleviated bycreation of entrained gases within the molten plastic. These entrainedgases will act as a lubricator by substantially changing the fluidity ofthe melt and decreasing the amount of injection pressure required toinject the molten plastic into the cavity. The entrained gases cause themolten plastic to be much more pliable and easier to manipulate. Also,the predicted volumetric difference between the volume of the moldcavity and the volume of a solid part molded by the cavity can be usedas a basis for premixing the molten plastic in the barrel at sufficientpressure to resist the volumetric shrink and eliminate the internalstresses.

By generating a certain desired amount of entrained gases and moisturewithin the molten plastic, a level of partial pressure of the entrainedgases and moisture may be established at which movement of thepressurized gas and moisture will be stopped. Also, the decomposition ofthe gases may be stopped and the gases may be forced to maintain astatic position by means of balanced surface tension forces. Inaddition, negative pressures in the solidifying plastic are eliminated.

The venting system shown in FIG. 2 creates the possibility ofmaintaining a constant gas pressure resistance in the cavity, whicheliminates uneven flow distribution of the molten plastic in the cavity.The molten plastic will be distributed substantially concentrically inthe cavity space with respect to the aperture 28. This provides theunique possibility for the melt to travel in the cavity and solidify inthe cavity under the same pressure characteristics at all sections ofthe part. This also eliminates the possibility of gases entrained withinthe molten plastic traveling to the surface of the shot of the plastic.The feeding rate of the molten plastic into the cavity is maintainedconstant in all areas of the cavity. This constant feeding rate alongwith the internal pressures created in the entrained gases and moistureprovide the advantage of increasing the cooling rates because of earlierpressurized contact of the molten plastic with the cavity walls. Thispressurized contact allows the molten plastic to cool more quickly as aresult of heat dissipation through the walls of the cavity. Increasedcooling rates result in substantial cycle time reduction which leads toconsiderable savings for the manufacturer.

Controlling the air pressure in the cavity of the mold provides thecapability of establishing a balanced molding environment for the moltenplastic. Processing the molten plastic under these conditions preventsdegradation and scission of the polymers which are normally chemicallyattacked by decomposition products in the presence of moisture.

As a result of the pressure balance between the air pressure in thecavity and the melt pressure, the development of surface tension in theplastic is avoided. Effectively, this balanced pressure system creates adirectional solidification of the plastic. In other words, the moltenplastic cools in a constant, straight line from the surface of themolten plastic to the center of the plastic. This directionalsolidification eliminates surfaces stresses, which lead to shrink, sinkand warpage of the part. The end result of this process is theproduction of a part which is free of shrink and sink, fully stressrelieved, and a nearly exact copy of the cavity surface. Furthermore,this process produces parts having strong mechanical properties andconfiguration stability in addition to enhanced structural integrity.

In accordance with the most preferred embodiment of the presentinvention, a method of injection molding for use with an injectionmolding machine is provided. The first step is to calculate a maximumstress to be experienced by a shot of plastic to be molded in theinjection molding machine 10 in accordance with the particular moldcavity configuration, the stress being a result of volumetric shrinkoccurring as the plastic is cooled in the cavity 30 of the mold 22.Assuming that the part to be molded is an elongate rod having arectangular cross-section, the following formulas apply. The maximumuniform load experienced by the part as a result of shrink is calculatedas follows: ##EQU2## where q is a uniform load per unit area, a is widthof the cavity, b is thickness of the cavity, h is height of the cavity,E is apparent modulus of elasticity of the plastic, and y is a shrinkagefactor of the plastic.

The uniform load calculation equations will vary, depending upon theconfiguration of the part and the plastic to be processed. Of course,these formulas may be programmed into the machine controller so that theoperator is only required to enter the properties of the plastic toprocess the machine.

A maximum mechanical stress to be experienced by the shot of plastic isthen calculated in accordance with the maximum uniform load: ##EQU3##where S (mechanical) is a maximum mechanical stress to be experienced bythe part, a is width of the cavity, b is thickness of the cavity, h isheight of the cavity, and q is the uniform load per unit area.

Again, the maximum mechanical stress calculation equations will varydepending upon part configuration.

A maximum thermal stress to be experienced by the shot of plastic isthen calculated in accordance with the following formula:

    S (thermal)=dT*L*E

where S (thermal) is a maximum thermal stress to be experienced by thepart, dT is a change in the temperature of the plastic between roomtemperature and the temperature at which the plastic is in a plasticrange of deformation, L is a thermal coefficient, and E is a modulus ofelasticity of the plastic.

Finally, the maximum stress to be experienced by a shot of plastic isdetermined to be the greater value of S (mechanical) and S (thermal).

The next step in the process is to pressurize a shot of plastic to afirst melt pressure as the plastic is plasticized in the barrel 16 ofthe injection molding machine, the first melt pressure beingsubstantially equal to the calculated maximum stress.

The cavity 30 is then pressurized to an air pressure which issubstantially equal to the first melt pressure. Injection of the shot ofplastic into the cavity is commenced after the air pressure has reachedthe first melt pressure. As the molten plastic enters the cavity 30, theair pressure in the cavity acts against the melt pressure in order toprovide a pressure balance for the molten plastic.

As the shot of plastic is injected into the cavity, the melt pressure onthe shot of plastic is increased from the first melt pressure to asecond melt pressure, while maintaining the air pressure within thecavity substantially constant. In addition, a substantially constantdifference between the air pressure within the cavity and the secondmelt pressure is maintained during a substantial portion of a period oftime in which the shot of plastic is being injected into the cavity.Reference to FIGS. 4 and 5 further illustrates this method. Referring toFIG. 4, the air pressure 50 in the cavity and the melt pressure 52 ofthe molten plastic are illustrated as a function of time. During theperiod of time t1, the air pressure 50 is built up to equal the meltpressure 52. The time period t2 is a relaxation time to allow the airpressure to equalize with the melt pressure. During t3, injection of themolten plastic into the cavity begins and the melt pressure increasesfrom the first melt pressure to the second melt pressure. During t4, thepressure difference between the second melt pressure and the airpressure 50 is maintained substantially constant, as shown in FIG. 5. Inthe t5 period, the melt pressure 52 is decreased from the second meltpressure to the first melt pressure, and the two pressures are equalizedduring the t6 period. At t7, the air pressure 50 in the cavity isreleased and the next shot of plastic is prepared.

The present invention contemplates that no specific pressure profilesare required for the air pressure or the melt pressure. The key to thisinvention is the preloading of the system and the maintenance of apressure difference between the air pressure and the melt pressure for asubstantial portion of injection. The pressure difference is generatedin accordance with the specific usage requirements in order to controlthe amount of dynamic pressure which is converted to static pressure,and the pressure difference may vary accordingly. Therefore, the airpressure and melt pressure may be decreased or increased in accordancewith any pressure profile, so long as the pressure difference betweenthe melt pressure and air pressure is maintained substantially constant.Furthermore, it is not a requirement that the air pressure be originallyset equal to the maximum stress calculation. Again, the key is thedevelopment and maintenance of a pressure difference between the airpressure and melt pressure as the molten plastic is injected into thecavity. Varying air pressure and melt pressure profiles are contemplatedunder the present invention.

A closed loop controller 40 is provided to monitor the first and secondmelt pressures and the air pressure in the cavity, and to producesignals for maintaining the first and second melt pressures.

Finally, the method is repeated by returning to the step of pressurizingthe next shot of plastic to a first melt pressure in the barrel 16.Accordingly, injection molded products are produced repeatedly.

It is further preferable to inject the shot of plastic into the cavityfrom the barrel 16 at relatively low rates. Manufacturers commonlyprovide suggested injection speeds within high and low speed values. Itis desirable to inject a molten plastic into the cavity in the lower 10%of rates suggested by manufacturers in order to decrease turbulence andmaterial degradation of the plastic. Similarly, manufacturers providehigh and low injection pressure values. It is desirable to inject theplastic into the cavity at an injection pressure in the lower 10% ofranges suggested by manufacturers. Filling the cavity at low injectionspeeds and low injection pressures avoids destruction and degradation ofthe polymer chains.

A second embodiment of the present invention provides a method ofinjection molding for use with an injection molding machine, comprising:(a) forming a plurality of vents 42 in a mold for use in the injectionmolding machine, the mold having a cavity formed therein, the ventsbeing in fluid flow communication with the cavity 30 of the mold to ventpressurized air from the cavity, while maintaining a substantiallyconstant air pressure in the cavity, according to the following formula:A=0.24241W*√T1/(C*P1), where A is a cross-sectional area of the vent, Wis discharge of air through the vent in pounds per second, c is acoefficient of flow, P1 is the air pressure in the cavity in pounds persquare inch, and T1 is a temperature in the cavity in degreesfahrenheit; (b) forming a channel 44 in by the mold in fluid flowcommunication with the vents 42; (c) sealing the mold to prevent leakageof pressurized air from the cavity and from the channel; (d) providingfirst and second valves 46,48 in selective fluid flow communication withthe channel 44 formed by the mold, the first valve 46 being selectivelymovable between a closed position wherein pressurized air is preventedfrom moving therethrough and an open position wherein pressurized air isallowed to enter the channel 44 therethrough, and the second valve 48being selectively movable between a closed position wherein pressurizedair is prevented from moving therethrough and an open position whereinpressurized air is allowed to discharge therethrough from the channel44; (e) calculating a maximum stress to be experienced by a shot ofplastic to be molded in the injection molding machine, the stress beingthe result of volumetric shrink occurring as the plastic is cooled inthe cavity of the mold; (f) pressurizing a shot of molten plastic in thebarrel 16 of the injection molding machine to a first melt pressure, thefirst melt pressure being substantially equal to the maximum stress; (g)moving the first valve 46 to the open position; (h) moving the secondvalve 48 to the closed position; (i) introducing pressurized air throughthe first valve 46 into the cavity 30 until the air pressure in thecavity is substantially equal to the first melt pressure; (j) moving thefirst valve to the closed position; (k) commencing injection of the shotof plastic into the cavity 30; (l) increasing the melt pressure of theshot of plastic to a second melt pressure; thus creating a pressuredifference between the second melt pressure and the gas pressure in thecavity; (m) maintaining the pressure difference between the second meltpressure and the air pressure in the cavity substantially constant for asubstantial portion of the period of time in which plastic is beinginjected into the cavity; (n) moving the second valve 48 to the openposition to release pressurized air from the channel; and (o) repeatingsteps (f)-(n).

It is understood that these steps need not necessarily be performedsequentially. Variations in the order of the steps provided in thismethod are contemplated as part of the present invention.

Reference to FIG. 4 provides a basis for description of the valves 46,48and the venting system as provided in the second embodiment of thepresent invention described above. Beginning with step (h) of the secondembodiment of the present invention, the first valve 46 is moved to theopen position and the second valve 48 is moved to the closed positionprior to the time period t1 of FIG. 4. During the t1 period, pressurizedair is introduced through the first valve 46 into the cavity 30 untilthe air pressure in the cavity is substantially equal to the first meltpressure. The first valve 46 is then moved to the closed position. Thefirst and second valves remain closed as the molten plastic is injectedinto the cavity. A pressure difference is then established andmaintained between the air pressure in the cavity and a second meltpressure for a substantial period of time (T4). The melt pressure andair pressure are then equalized (T5,T6), and then the second valve 48 ismoved to the open position in order to release pressurized air from thechannel (T7).

The purpose of this closed venting system is to provide a moldingenvironment for the molten plastic wherein pressures acting upon eachindividual gas molecule are balanced so that very little movement of thegas occurs within the plastic between adjacent molecules. This balanceprevents gas molecules from moving toward the surface of the plastic orincorporating with other gas molecules to form larger voids or cells.

One skilled in the art will appreciate the utility of adding chemicalblowing agents to the molten plastic. Chemical blowing agents are usefulin controlling the amount of gas decomposed and entrained in the meltduring plastification. These blowing agents generate inert gases whenheated. The gases create voids in the material which can lead tosubstantial weight reduction of the part, and the pressure of thesevoids may be used to help fill the cavity and to pack out the plasticagainst the walls of the cavity.

Referring to FIGS. 1-3, a third embodiment of the present inventionprovides a mold 22 for use in an injection molding machine 10. The moldhas a front half 24 and a back half 26, the front half 24 having anaperture 28 formed therethrough for receiving molten plastic from theinjection molding machine. The front half 24 and the back half 26cooperate to form a cavity 30 therebetween. The cavity 30 is in fluidflow communication with the aperture 28 to receive the molten plastictherefrom. A plurality of vents 42 are formed in the back half of themold, as shown in FIG. 2. The vents 42 have first and second endsthereof. The first end of each of the plurality of vents is in fluidflow communication with the cavity 30. The vents are configuredaccording to the following formula to maintain a substantially constantair pressure in the cavity 30 as the cavity is being filled withplastic: A=0.24241*W*√T1/(C*P1), where A is a cross-sectional area of avent, W is discharge of pressurized air through the vent in pounds persecond, C is a coefficient of flow, P1 is the air pressure in the cavityin pounds per square inch, and T1 is a temperature in the cavity indegrees Fahrenheit.

A channel 44 is formed in the back half of the mold. The channel 44 isin fluid flow communication with the second end of each of the pluralityof vents 42. The cavity 30, vents 42 and channel 44 are shown in FIG. 3.A pair of valves 46, 48 are in selective fluid flow communication withthe channel 44. The first valve 46 is adapted to selectively receivepressurized air from a pneumatic line 34 to provide pressurized air tothe channel 44. The second valve 48 is adapted to selectively allowdischarge of pressurized air from the channel 44.

A rubber seal 38 is provided in the back half of the mold andcircumscribes the cavity and the channel, and is positioned between thefront half and back half to prevent discharge of pressurized air fromthe cavity and from the channel as the cavity is being filled withmolten plastic. Upon injection, the front half and back half of the moldhave a tendency to separate slightly. The rubber seal 38 preventsleakage of pressurized air from the cavity when this separation occurs.

A very similar counterpressure technology may be applied to the processof aluminum casting. Referring to FIG. 11, a comparison is illustratedbetween the injection pressure profile of the prior art in aluminumcasting, and an air pressure and injection pressure profile inaccordance with the present invention. FIG. 11 demonstrates a timeversus pressure profile. Along the time axis, point A is the time atwhich the plunger of the aluminum casting apparatus begins to moveforward (as illustrated in FIG. 6). Point B represents the period atwhich molten aluminum begins entry into the mold cavity (as illustratedin FIG. 7). Point C represents the point in time at which the moldcavity is substantially filled with molten aluminum (as illustrated inFIG. 8). The pressure line illustrations beyond point C illustrate thebuilding of pack pressure to pack out the molten aluminum within themold cavity.

More specifically, with reference to FIG. 11, line 100 illustrates theinjection pressure in accordance with a prior art aluminum castingprocess. Between points A and B, while the plunger is moving along thesleeve, pressure acting upon the molten aluminum is extremely low. Thislow pressure arrangement provides ripe conditions for entrapment ofgases, wave formation, and emergence of hydrogen gas from the melt whichcauses porosity in the melt. Between points B and C, the molten aluminumis injected into the cavity. Because the aluminum casting apparatus isunable to maintain a high pressure acting upon the aluminum duringinjection, conditions remain for formation and maintenance of gasporosity and void formation. At these low pressures, the solubility ofhydrogen within the molten aluminum is very low, thus creating anenvironment for formation of porosity and voids and for the separationand emergence of hydrogen from the solidifying melt. These low meltpressure conditions in the cavity create tremendous problems within thefinal part as cast due to stresses, volumetric deficits and negativepressures created within the molten aluminum as it solidifies anddevelops porosity and voids, which adversely effect metallic bonding. Avolumetric deficit is simply the volumetric difference between theliquid and solid state of a substance. Volumetric deficits can resultfrom uneven cooling rates, and will create a vacuum-type void wheninside the part, and will create a sink mark when formed outside thepart. Stresses created from these volumetric deficits can lead tocracking of other structural problems. Furthermore, beyond point C, ittakes a substantial period of time for the aluminum casting apparatus tobuild a sufficient pack pressure to pack out the aluminum within themold cavity. This is mainly due to the fact that the aluminum castingapparatus is unable to maintain a high pressure acting upon the moltenaluminum while the mold cavity is being filled. Accordingly, since thepack pressure starts out at a low point, it takes longer to build up thehigh pack pressure which is required.

However, according to the present invention, higher melt pressureconditions are created within the molten aluminum throughout the processin order to increase the solubility of hydrogen within the moltenaluminum, decrease wave formation, decrease entrapment of gases andformation of porosity and voids, thus providing a part which issubstantially stress relieved and free of gas porosity and voids.Accordingly, mechanical strength and structural integrity of the part isgreatly enhanced.

Referring to FIG. 11, an air pressure 104 is established within the moldcavity and within the sleeve prior to injection. Between points A and B,the air pressure decreases the formation of waves which leads to reducedentrapment of gases, and increases the melt pressure of the moltenaluminum which increases the solubility of hydrogen within the melt,thereby eliminating hydrogen void problems. The air pressure providesthe capability of maintaining a higher melt pressure throughout theprocess. The large pressure surge after point B in the prior art inorder to build the pack pressure causes turbulence within the moltenaluminum and encourages "coasting" of waves back and forth within themolten aluminum in the mold cavity. These turbulent conditions causeproblems within the molten aluminum as it solidifies. Between points Band C, during injection, the static melt pressure is maintained at ahigher level than prior art systems, thereby creating a better castingenvironment and eliminating negative melt pressures duringsolidification at the time of filling of the cavity. Furthermore, beyondpoint C, the aluminum casting apparatus may raise the pressure up to asufficient pack pressure level in a much shorter period of time thanrequired by the prior art, thereby decreasing cycle time.

FIG. 9 is a graphical illustration of the injection pressure 102, shotspeed 103 and cavity air pressure 104 versus time in accordance with thepresent invention, and corresponding with FIG. 11. As shown in FIGS. 9and 11, the injection pressure build-up time is much quicker with thepresent invention (102) than with the prior art (100). FIG. 10 is agraphical illustration of the pressure difference between the airpressure in the cavity and the melt pressure versus time, in accordancewith the cavity air pressure 104 development and injection pressure 102development illustrated in FIG. 9.

A more detailed description of the present invention is provided withreference to FIG. 6. FIG. 6 illustrates a schematic view of an aluminumcasting system 110 in accordance with the present invention. Thealuminum casting apparatus 112 includes a pair of platens 116 with amold 118 supported therebetween. A mold cavity 120 is formed within themold 118 in the configuration of a part to be cast. Molten aluminum ispoured into the sleeve 122 through the feed throat 124 from the spout128.

Once molten aluminum has been poured into the sleeve 122 through thefeed throat 124, a sealing collet 130 is placed over the feed throat124. The collet 130 includes graphite seals 132 in order to provide anairtight environment for the sleeve 122. Air pressure is provided to themold and the sleeve from a pneumatic system 156 through the air pressurevalve and venting device 136. The pressurized air travels into the moldfrom the pneumatic system to the venting device 136, through theovershot well 138 and past the vents 139. Pressurized air travelsthrough the mold cavity past the gate 141, and into the sleeve 122. Themold 118 is sealed airtight by the rubber/graphite seals 140. An ejectorpin 144 is provided with a pressure transducer 146 therein to sensepressure in the mold cavity.

A plunger 148 is provided in communication with hydraulic injection unit150 to provide sufficient injection pressure, in accordance with apredetermined pressure profile for the specific application, to theplunger 148 to move the molten aluminum from the sleeve 122 into themold cavity 120. A controller 152 is provided in communication with ahydraulic pressure transducer 143. The hydraulic injection unit 150monitors the hydraulic injection pressure through the transducer 143. Alinear transducer 154 measures the speed and position of the plunger148, and is in communication with the controller 152 to provide plungerspeed information thereto. The controller 152 is also in communicationwith the cavity pressure transducer 144, which operates in cooperationwith the ejector pin 146 for direct measurement of pressure in thecavity. The pneumatic system 156 provides pressurized air to the airpressure valve and venting device 136 and sends pressure information tothe controller 152.

The controller 152 monitors the sensed pressure from the cavity pressuretransducer 144, the linear transducer 154, and the hydraulic injectionpressure from the hydraulic injection unit 150, as sensed by thehydraulic pressure transducer 143. The controller makes a calculation todetermine the melt pressure of the molten aluminum throughout theprocess. A prescribed temperature profile, including a pressuredifference between the melt pressure and air pressure is provided to thecontroller. The controller 152 controls the hydraulic injection pressureand the plunger speed in order to maintain the melt pressure at desiredlevels in accordance with the prescribed pressure difference profile.

The first step performed is to calculate a maximum stress for the moldcavity configuration, as described above with reference to the injectionmolding process. Next, an air pressure calculation for the sleeve isperformed. A sample calculation for a linear gate is demonstrated withthe following formula: ##EQU4##

This calculation determines the amount of air pressure needed in orderto eliminate the formation of waves in the molten aluminum in the sleeveas the plunger 148 moves at a certain speed. Different gateconfigurations would require different calculations. "Linear gate speed"is the speed at which the molten aluminum moves through the gate. Lineargate speed values can be readily obtained from charts provided byaluminum material suppliers, and are charted against pressure for thedifferent types of aluminum. The "discharge coefficient" is a ratio ofthe pressure of the melt in the mold cavity and the pressure of the meltwithin the gate. The discharge coefficient is also available from chartsprovided by materials manufacturers. It is also commonly available fromAmerican Foundryman's Society publications. Additional backgroundinformation on discharge coefficient and linear gate speed, and how tocalculate for different gate configurations may be found in "PRESSUREDIECASTING Part 2, The Technology of the Casting and the Die", D. F.Allsop, copyright 1983, and "Cating Die Casting Dies", Van Kens, NADCA,EC-514. These publications are available in libraries and through theNorth American Die Casting Association.

An air pressure is then established in the sleeve 122 which is equal tothe larger number of the maximum calculated stress or the calculated airpressure needed to eliminate wave formation. Of course, the air pressurein the sleeve could be set at a greater level. In fact, as the plungermoves, the air will be compressed, which will increase the air pressure.

Referring back to FIG. 11, no closed loop control is required betweenpoints A and B because the system is in transition from static todynamic. When the system reaches point B, static pressure control isrequired, and a pressure difference is established between the airpressure and the melt pressure, the pressure difference being equal tothe maximum stress calculated to occur during injection andsolidification of the part as it is constrained within the mold cavity.

Still referring to FIG. 11, the injection pressure 102 is slightlyhigher than the air pressure 104 in the sleeve when the plunger 148begins moving because the density of the aluminum is higher than thedensity of the air in the sleeve, so the molten aluminum accepts moremomentum and develops a higher pressure.

When the system reaches point B, as displayed in FIG. 7, the moltenaluminum has reached the gate 141. The dynamic melt pressure of themolten aluminum is then calculated in accordance with the followingformula: ##EQU5## where cd is the discharge coefficient at the gate. Thegate velocity is a function of the flow rate and the area of the gate.The flow rate is provided by the hydraulic injection unit, and is afunction of the hydraulic injection pressure and the injection speed.

The controller 152 makes this calculation approximately every 2milliseconds and adjusts the hydraulic injection pressure or the plungerspeed in order to maintain a desired pressure difference between thedynamic melt pressure and the air pressure in the cavity as the moltenaluminum is injected into the cavity.

The maintenance of a pressure difference between the dynamic meltpressure and the air pressure in the cavity maintains a static pressurewithin the molten aluminum as it is injected into the mold cavity. Thisstatic pressure provides venting power to the melt. This venting powerallows the melt to vent, fill, and pack out any ribs or bosses whichexist lateral to the flow direction of the molten metal. Accordingly,the molten metal fully packs out the mold cavity as it moves through thecavity. The static pressure created by the pressure difference alsoprevents trapping of gases from mold release spray and trapping ofmoisture from mold release spray. In addition, the static pressuremaintains a high level of solubility of hydrogen within the moltenaluminum, thereby preventing the formation of hydrogen voids and gasporosity within the aluminum.

"Static pressure" is the pressure of the melt acting in all directions,while the "dynamic pressure" is the pressure of the melt in the flowdirection. For example, with the present invention, if the dynamic meltpressure is equal to the air pressure in the cavity, then the melt willnot flow into the cavity. However, if the air pressure is less than thedynamic melt pressure, then the melt will move into the cavity, and theresult of the air pressure pushing against the oncoming melt front is tocreate a "static" pressure in the melt which acts in all directions,including laterally to the flow direction. The air pressure in thecavity acts to compress the melt as it flows into the molt cavity, whichcreates a general internal static pressure in the molten material, asthe material wants to expand against this air pressure resistance. Thisstatic melt pressure provides pressures or forces acting laterally tothe flow direction which helps to force the molten material against thewalls of the mold cavity in order to "pack out" the material from insideso that it is fully pressed against the walls of the cavity. The staticpressure also gives the molten material "venting power" which is theability of the molten material to enter into lateral indentations (orribs) in the mold cavity and push any air or other gases out of thatindentation (i.e., "vent out" the area) so that the molten material canfully fill the indentation (or rib) and "pack out" that area with moltenmaterial as the molten material flows past the indentation as a resultof its dynamic pressure. The static pressure also counteracts volumetricshrinkage forces for prevention of sink marks or internal voids.

There are numerous advantages provided by the present invention. Thepossibility for a choice of proper composition and pressure of the gasabove the melt in the sleeve is provided. This makes possible thedissolving or elimination of trapped gases and maintenance of dissolvedgases on the basis of the law of mass exchange (m=c×P). For example, asimple two component system is the solution of a gas dissolved in aliquid. There are two phases, the solution and the gas above it. Then,by the phase rule, two variables must be given to describe theequilibrium between the phases; these variables are temperature andpressure. When temperature is held constant, the effective pressure indetermining the mass of gas dissolved is given by Henry's Law:Mass=Constant×Pressure. If the two components react, more gas willdissolve. Additional background information regarding Henry's Law may befound in Grolier's Encyclopedia in Grolier's Electronic Publishing,1992. The amount of hydrogen dissolved in the melt is a direct functionof the pressure applied on the melt. The formation of waves in thesleeve and cavity causes a wide range of pressures which increase anddecrease the solubility of hydrogen in the melt. Ideally, it isdesirable to dissolve as much hydrogen as possible within the melt.Undissolved hydrogen will incorporate in voids, cause porosity, anddecrease structural integrity by interfering with metallic bonding.

At a high partial pressure of gas within the sleeve, hydrogen solubilityincreases. The flattening of wave formation decreases the possibilitiesfor entrapment of gases or air within the molten aluminum. The airpressure in the sleeve decreases movement and further incorporation ofexisting hydrogen within the molten aluminum. The air pressure alsohelps to develop a desirable static pressure within the melt.

High partial gas pressures within the mold cavity maintain high hydrogensolubility within the molten aluminum and hydrogen separation from thesolidifying melt is decreased, as well as the formation of gas porosityand voids. Any inert gas, such as air, hydrogen or argon may be used inthe mold cavity. Protective gases can be used for metals with a higherlevel of affinity to form oxides.

The air pressure acting against the front of the rising metal throughthe feeder heads provides improved "feeding" of the casting during thesolidification. "Feeding" is a term used to describe the process bywhich the crystal structure of the solidifying aluminum is "fed" withmolten aluminum at the right place, at the right time and at the rightpressure in order to avoid the formation of voids and to improvemetallic bonding during solidification. The air pressure provides thepossibility to establish and maintain a constant pressure differencebetween the melt pressure and air pressure, which improves feeding. Thispressure difference is controllable by the machine controller 152.

The grain uniformity and mechanical properties of thick walls of thecasting will be identical with those of thin walls using this process.In areas of abrupt changes between thin and thick walls, no unsoundstructure is obtained. The casting has higher configuration stabilityand strength because, ideally, there are no spots with substantialcrystalline differences or voids from the incorporation of unwantedgases. The static pressure of the molten aluminum improves theinfiltration of the melt between the crystals which form from the cavitysurface inward, which decreases or eliminates conditions under whichaxial porosity occurs.

When the solidifying crystals are in contact but still include someliquid metal, the entire structure is mobile and subject to deformation.Accordingly, providing static pressure in the melt to these solidifyingcrystals, which include liquid metal, even at low pressures, leads tomicro plastic deformation and to the elimination of conditions whichlead to natural porosity.

This type of closed loop control of the injection from a createdpressure difference provides precise control of the filling rate of themold cavity. This method can also reduce the size of the riser 158, thusincreasing yield. Furthermore, the improved pressure balance reducescore and mold deformation and breakage, thus improving the life of themold. It also improves the effectiveness of overshot wells (feederheads), or else reduces the need for the overshot wells. In addition,the laminarity of the flow is increased, and "coasting" of waves backand forth within the cavity decreases, which results in less turbulentsolidification conditions.

A major advantage of this invention is that the maintenance of thepressure difference between the melt pressure and the air pressureeliminates the "way resistance" or mold resistance effects of varyingpart configurations and gate configurations. This greatly increases thetechnological possibilities for resolving casting problems in criticalsections of the casting. For example, if a pressure drop is created by athick walled section of a mold cavity, the air pressure and meltpressure may be maintained at higher levels in order to reduce orcompletely eliminate the pressure drop effects of the thick section.Furthermore, the present invention maintains a higher melt pressurethroughout the process. Accordingly, a high pressure is maintained whilethe temperature is still high in the molten aluminum, which providesbetter infiltration and better settling conditions within thesolidifying crystals, in comparison to the prior art.

The injection power curve shown in FIG. 12 further illustrates theadvantages of the present invention. Using prior art systems, theinjection pressure or melt pressure and flow rate are limited by thepower curve line 170. At point X₁, the mold cavity is full and maximuminjection pressure is received from the machine. At X₂, there is no moldresistance, and maximum flow rate is achieved. A line connecting thesetwo points defines the limit of power available in this system. Forexample, if we calculate the dynamic melt pressure P to be P₁, the flowrate is limited to Q₁ in accordance with the prior art systems. However,this flow rate may be too low to support the feeding of solidifyingcrystals during filling of the mold because cooling occurs too quickly.Therefore, in accordance with the present invention, if we add an airpressure to the other side of the melt, the power curve line 170 is nolonger limiting and the mold cavity can be effectively fed at a highflow rate using a high pressure. Effectively, the static pressurecreated gives the possibility to exceed the maximum machine power curvein accordance with the prior art and to achieve pressures and flow rateswithin the cross-hatched area of FIG. 12.

Accordingly, the present invention gives the possibility to operate analuminum casting apparatus at a high injection pressure, a high flowrate, and it facilitates the use of larger gate sizes.

Of course, this invention is also applicable to aluminum castingmachines employing a vertical sleeve or vertical injection unit withhorizontal die clamping.

While the best modes for practicing the invention have been described indetail, those familiar with the art to which this invention relates willrecognize various alternative designs and embodiments for practicing theinvention as defined by the following claims.

What is claimed is:
 1. A method of injecting molten metal from a meltholder into a mold cavity defined within a mold in a metal castingapparatus, comprising:supplying pressurized gas to the mold cavity andmelt holder to pressurize gas in the cavity and molten metal in the meltholder prior to injection; generating a melt pressure in the moltenmetal sufficient to inject the molten metal into the cavity after thegas in the cavity is pressurized; monitoring the melt pressure and thepressure of gas in the cavity as the melt moves from the melt holderinto the mold cavity; and providing a closed loop controller to maintaina predetermined controllable pressure difference between the meltpressure and the pressure of gas in the cavity throughout a substantialportion of a period of time during which molten metal is being injectedinto the cavity to fill the cavity.
 2. The method of claim 1, whereinthe maintenance of said predetermined pressure difference includes thesteps of:calculating a maximum stress to be experienced by thesolidified metal in the metal casting apparatus; and maintaining saidmelt pressure substantially equal to said calculated maximum stressduring a substantial portion of a period of time during which the moltenmetal is injected into the cavity to fill the cavity.
 3. A method ofimproving venting power of molten metal as it is injected from a meltholder into a mold cavity formed within a mold in a metal castingapparatus, the cavity including small indentations formed therein whichexist lateral to the flow direction of the molten metal, the moldincluding vents formed in communication with the cavity, the moltenmetal having a dynamic pressure as it moves into the mold cavity and theventing power being the molten metal's ability to force any existingcavity air or other gases through the vents of the mold cavitysufficiently to allow the metal to pack out against the walls of themold cavity, the method comprising:providing pressurized gas to the moldcavity prior to injection; and maintaining a pressure difference betweenthe dynamic melt pressure and the pressure of gas in the cavity tocreate a static pressure in the molten metal, whereby to improve theventing power of the molten metal to allow said molten metal to vent,fill and pack out any said indentations as the molten metal movesthroughout the cavity.
 4. A method of injecting molten metal from a meltholder into a mold cavity defined within a mold in a metal castingapparatus having a movable plunger for injection, and wherein the moltenmetal experiences pressure transitions in response to plunger movementand varying resistance to flow as the molten metal is injected into thecavity, comprising:supplying pressurized gas to the mold cavity and tothe melt holder prior to injection to pressurize gas in the cavity andmolten metal in the melt holder sufficiently to provide a preloadedstatic system in which the gas in the mold cavity is pre-pressurized andmolten metal in the melt holder is pre-pressurized with static pressureprior to injection; generating a melt pressure in the molten metalsufficient to inject the molten metal into the cavity after the gas inthe cavity is pressurized; monitoring the melt pressure and the pressureof gas in the cavity as the melt moves from the melt holder into themold cavity; and providing closed loop pressure control for maintainingpressure differences between the monitored melt pressure and themonitored pressure of gas in the cavity in accordance with apreprogrammed pressure profile for injecting the molten metal into saidcavity, whereby to maintain the static pressure in the molten metalduring injection.
 5. A method of injecting molten metal from a meltholder into a mold cavity defined within a mold in a metal castingapparatus, comprising:supplying pressurized gas to the mold cavity andto the melt holder prior to injection to pre-pressurize the cavity andthe molten metal sufficiently to provide a preloaded static system inwhich the gas and the molten metal are both pre-pressurized prior toinjection; generating a melt pressure in the molten metal sufficient toinject the molten metal into the cavity against the pressurized gas inthe cavity; and maintaining the melt pressure and the pressure of gas inthe cavity at desired levels during injection.